This invention relates to a method of wastewater purification with recovery of heavy metals contained therein, and more particularly to a method of coagulating metal containing wastes from industrial waste streams with subsequent recovery of those metals and reuse of the coagulant.
Metal plating and metal finishing are commonly used processes to improve surface properties of metallic and nonmetallic articles. Electroplating is the most common and important of the various metal plating processes. Metals commonly used in these processes include nickel, copper, iron, lead, chromium, zinc, gold, silver, cadmium and tin. Such metals are commonly referred to as "heavy metals" in part due to their potentially toxic effects. As used in this specification, the term "heavy metals" will encompass any metal known to have a potential toxic effect on animal or plant life and which can be removed by precipitation or flocculation.
Waste water from the various industrial metal plating and finishing operations amounts to several billion gallons per year in the United States alone. Major sources of these wastes include drag-out losses carried into rinse waters by the plated piece or the rack holding it, solution dumping of spent, spoiled, or obsolete solutions (rarely done unless remedial measures fail), and tank leakage losses. Although trace amounts of metals in water supplies are not harmful, their presence in concentrations of as little as 0.05 ppm in some cases can have potentially toxic effects on animal and aquatic life. Additionally, unlike organic pollutants which eventually degrade to harmless materials, metals are not subject to degradation and may pass along food chains where they tend to accumulate, reaching concentrations in excess of that which originally existed in the air or water.
For this reason, the U.S. Public Health Service has established drinking water standards setting maximum acceptable concentrations of metals in water supplies. A more detailed and comprehensive listing of the toxic effects of metal containing waste streams can be found in an Environmental Protection Agency publication entitled "Water Quality Criteria Data Book, Volume 3, Effects of Chemicals on Aquatic Life" (Pub. No. 18050 GNV 05/71). As can be seen, there is a need for an effective means for removing such heavy metals from waste streams to avoid pollution of drinking water supplies. Of course, this need has been recognized, and numerous attempts have been made to treat effectively metal containing waste streams. These methods have included dilution and discharge into sewers or water courses; evaporation, dialysis, and electrodialysis; reverse osmosis; neutralization-precipitation; and ion exchange.
Of these methods, neutralization-precipitation has been among the most commonly used. Such treatment involves the addition of basic materials such as sodium or calcium hydroxide or the like to a metal containing waste stream to precipitate heavy metals as their insoluble hydroxides. In many instances, other chemical additives such as settling aids or coagulants are also added. In the prevalent industrial practice, the precipitated heavy metal flocs (hydroxides) are then separated from the stream and then disposed of on landfill sites. However, many disposal sites are inadequate because of potential ground and surface water contamination caused by resolubilization of the metals. Even carefully selected landfill sites may present long term disposal problems according to the Environmental Protection Agency's Hazardous Waste Management Division. See, "Assessment of Industrial Hazardous Waste Practices; Electroplating and Metal Finishing Industries--Job Shops" (Pub. No. PB-264349, September 1976). Moreover, neutralization and precipitation of heavy metals consumes large amounts of increasingly expensive chemicals and generates large volumes of sludge to be disposed of on scarce landfill sites.
Accordingly, the need still exists in the art for an efficient and relatively inexpensive method of removing heavy metals from waste streams which does not generate large volumes of sludge or other disposal problems.